1) Is it true?
2) Where I can read mostly latest info about VDP research? forum/doc?

Yes it's a side effect of how the highlight function works. Normally for sprites using palette 3, a sprite pixel value of $0E enables the highlight effect.
Now for whatever reason, possibly a bug, a sprite pixel value of $0E for any other palette (0,1,2) is also displayed at the normal brightness level.

All the current VDP findings are buried in the spritesmind forum postings. Nemesis is working on some comprehensive VDP documentation currently, and you can also look at the Genesis Plus GX source code which has really good VDP emulation thanks to Eke's hard work.